Conventionally, in a vehicle, a mechanical driving system is replaced by a system that electrically drives with a motor in order to satisfy requirements of conserving space, improving an assembling process, and improving controllability. For example, JP-A-2004-23890 (corresponding to US 2003/0222617 A1, referred to as a patent document No. 1 hereafter) discloses a system in which a position switching mechanism of an automatic transmission of a vehicle is driven with a motor. The mechanism includes an encoder that outputs a pulse signal at each predetermined angle in synchronization with a rotation of the motor. When a position is switched, the motor is rotated to a target rotational position (target count value) on the basis of a count value of the pulse signal of the encoder (hereafter, referred to as an encoder count value) so as to switch the shift position to a target position.
Each time the target rotational position is changed, the above-described system performs a feedback control in which the system rotates the motor to the target rotational position by sequentially switching a current-supply phase of the motor on the basis of the encoder count value.
If a current supply to a winding of a phase corresponding to the target rotational position is continued after the feedback control ends, the motor can be held at the target rotational position due to electromagnetic force. However, in this configuration, if a stopping time of the motor is long, the current supply to the winding of the same phase is continued for a long time, and the winding may be overheated. Thus, the current supply to the winding is stopped while the motor is stopped so as to restrict overheating of the winding.
However, if the current supply is stopped while the motor is stopped, the electromagnetic force to hold the motor at the target rotational position (the rotational position at the end of the feedback control) disappears. Thus, the rotational position of the motor may be displaced from the target rotational position, and the motor may fail to be normally rotated to the target rotational position, for example, the motor may step out, or the motor may reverse in a direction opposite from the target rotational position, when the next feedback control starts.
In order to solve the above-described issues, in the invention disclosed in the patent document No. 1, a stopping and holding process, in which electric current is supplied so as to stop and hold the motor, is performed for a predetermined current-supply holding time at the start or the end of the feedback control. Accordingly, step-out or reverse at the start of the feedback control can be restricted while restricting overheating of the winding.
According to a study by the inventors, when the stopping and holding process is performed for the predetermined current-supply holding time, a current-supply holding time required for bringing the motor to a stopping and holding state changes with a rotation speed of the motor just before the stopping and holding process. For example, when the rotation speed of the motor just before the stopping and holding process is high, the current-supply holding time required for bringing the motor to the stopping and holding state is long. On the other hand, when the rotation speed of the motor just before the stopping and holding process is low, the current-supply holding time required for bringing the motor to the stop holding state is short.
However, the patent document No. 1 does not take into consideration influence of the rotation speed of the motor just before the stopping and holding process and the current-supply holding time is set to a predetermined fixed value. Thus, the current-supply holding time needs to be set to a value longer than or equal to the maximum value required for bringing the motor to the stopping and holding state in anticipation of the worst case, that is, a case in which a time required for bringing the motor to the stop holding state is the longest. Thus, in most cases, the current supply holding time is longer than necessary, an execution time of the stopping and holding process is longer than necessary, and power consumption is more than necessary.